In-bed laying underwater pipelines is commonly known as “underground laying”, and comprises laying the pipeline along a given path on the bed of the body of water; breaking up a ground mass along the path to a given depth; digging a trench or generally removing the broken ground mass, which normally coincides with the ground-breaking stage; and possibly burying the pipeline.
More specifically, currently used known techniques comprise removing the broken ground mass to form a trench in the bed of the body of water; and lowering the pipeline into the trench. The pipeline may later be covered over with the removed ground mass to fill in the trench and bury the pipeline.
Underwater pipelines carrying hydrocarbons are normally laid completely or partly underground for various reasons, some of which are discussed below. Underwater pipelines are normally laid underground close to shore approaches, in relatively shallow water, to protect them from damage by blunt objects, such as anchors or nets, and are sometimes laid underground to protect them from natural agents, such as wave motion and currents, which may result in severe stress. That is, when a pipeline is laid on the bed of a body of water, it may span two supporting areas of the bed, i.e. a portion of the pipeline may be raised off the bed; in which case, the pipeline is dangerously exposed to, and offers little resistance to the movements induced by, wave motion and currents. Underground laying may also be required for reasons of thermal instability, which result in deformation (upheaval/lateral buckling) of the pipeline, or to protect the pipeline from the mechanical action of ice, which, in very deep water, may result in scouring of the bed.
To avoid damage, the pipeline often need simply be laid at the bottom of a suitably deep trench dug before laying (pre-trenching) or more often after laying the pipeline (post-trenching). At times, the protection afforded by the trench and eventual natural backfilling of the trench is not enough, and the pipeline must be buried using the broken ground mass removed from the trench, or any available ground mass alongside the trench.
The depth of the trench is normally such that the top line of the pipeline is roughly a meter below the surface of the bed, though severe environmental conditions may sometimes call for deeper trenches (of several meters). Trenching and backfilling are performed using digging equipment, and post-trenching (with the pipeline already laid on the bed) is the normal practice, to dig and backfill the trench in one go.
A first method of underground laying underwater pipelines is described in Patent Application WO 2005/005736. This is a post-trenching method comprising the steps of breaking up a ground mass in the bed to open the way; and drawing a huge plough along the opening to form a trench in which to lower the pipeline.
A second method of underground laying underwater pipelines is described in Patent Application WO 2004/016366, in which the method comprises breaking up a ground mass in the bed; clearing a trench using pumps installed on a support vessel; lowering the pipeline into the trench; and filling the trench using the pumps on the support vessel. In other words, the mass of broken material is first sucked out and then released, downstream from the suction area, by the pumps on the support vessel; and the trench extends between the suction area and the release area.
Both the above methods have the drawback of being highly energy-intensive, though for different reasons. And, in both cases, energy consumption increases exponentially alongside an increase in trench depth.